#include<bits/stdc++.h>
#define ll long long
#define ull unsigned long long
using namespace std;

const ll N = 1e6 + 10, INF = 0x3f3f3f3f;

ll a, b, x, y;

void solve()
{
    cin >> a >> b >> x >> y;
    if (a == b) { cout << 0 << '\n';return; }

    ll ans = 0;
    if (a < b) {
        while (a < b) {
            if ((a & 1) == 0) ans += min(x, y);
            else ans += x;
            a++;
        }
    } else if ((a ^ 1) == b) ans += y;
    else ans = -1;

    cout << ans << '\n';
}

signed main()
{
    // ios::sync_with_stdio(0);
    // cin.tie(0);cout.tie(0);
#define ONLINE_JUDGE
#ifndef ONLINE_JUDGE
    std::istringstream in(R"()");
    std::cin.rdbuf(in.rdbuf());
#endif
    ll T = 1;
    cin >> T;
    for (ll i = 1;i <= T;i++) {
        solve();
    }
}